Supercomputer Ready to make ALMA a Powerful Telescope

One of the most powerful supercomputers in the world has now been fully installed and tested at its remote, high altitude site in the Andes of northern Chile. This marks one of the major remaining milestones toward completion of the Atacama Large Millimeter/submillimeter Array (ALMA), the most elaborate ground-based telescope in history. The special-purpose ALMA correlator has over 134 million processors and performs up to 17 quadrillion operations per second, a speed comparable to the fastest general-purpose supercomputer in operation today.

The ALMA correlator’s 134 million processors will continually combine and compare faint celestial “signals” received by as many as 50 dish-shaped antennas in the main ALMA array, enabling the antennas to work together as a single, enormous astronomical telescope. The correlator can additionally accommodate up to 14 of the 16 antennas in the Atacama Compact Array (ACA), a separate part of ALMA provided by the National Astronomical Observatory of Japan (NAOJ), for a total of 64 antennas . In radio telescope arrays, sensitivity and image quality increase with the number of antennas.

Funded by the US National Science Foundation (NSF), and designed, constructed, and installed primarily by the National Radio Astronomy Observatory (NRAO), the ALMA correlator is a critical component in a radio telescope system that astronomers are already using to make new discoveries about how planets, stars, and galaxies form. Unlike optical telescopes, which observe visible light emitted by stars, ALMA explores a region of the spectrum of invisible light, the millimeter and sub-millimeter wavelength realm.

When observing, ALMA’s antennas point at the same celestial object in the sky, gathering faint radio waves. Before astronomers can make detailed images or do other analyses, the information collected by dishes separated by as much as 16 kilometers must be extensively computer processed.

The ALMA correlator performs the first critical steps in this data processing. To make the entire system work as a single telescope, the information collected by each antenna must be combined with that from every other antenna. At the correlator’s maximum capacity of 64 antennas, there are 2,016 antenna pair combinations, and as many as 17 quadrillion calculations every second.

Submillimeter-wavelength (0.3 - 1.0 mm) astronomy is perhaps the last wholly unexplored wavelength frontier. Why? Submillimeter ("microwave") astronomy is technically very difficult due to the sheer complexity of the instrumentation and to the "opaqueness" of the atmosphere in microwave light.

The Submillimeter (a.k.a. "microwave") frequency band lies between the regions easily observed by radio telescopes and optical telescopes. It comes as no surprise, then, that submillimeter astronomy borrows techniques used by both optical and radio astronomers. The visual appearance and operation of the telescope is that of a radio telescope, although it is sheltered in an enclosure like an optical telescope. Continuum-mode observations are done using specialized heat-sensing detectors called bolometers, which stem from infrared techniques. Spectral-line measurements, incorporating very high wavelength-resolution, use heterodyne receivers somewhat resembling those found in lower-frequency radio receivers. Such high frequency receivers (approaching 1 Terahertz, about 10,000 times higher frequency than the average FM radio!) are very difficult to manufacture, however. The recent production of sensitive receivers for astronomical purposes (at the University of Arizona (SORAL), among other places) has led recently to the opening of this wavelength band for the first time.

Ground-based submillimeter observations are difficult for another reason: the opaque-ness (opacity) of the atmosphere at microwave wavelengths.

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At submillimeter wavelengths, ambient atmospheric water vapor will absorb (block) incoming light. At low elevations, where most water vapor resides, the atmosphere is very opaque at submillimeter wavelengths; the abundant water vapor absorbs any incoming submillimeter photons before they can reach the telescope. At higher elevations, however, the water content decreases substantially. By minimizing the atmospheric water vapor, one improves the transparency of the atmosphere and makes astronomical observations possible. It is for this reason that infrared and submillimeter observatories are built as high as possible; by being above some of the atmosphere, the radiation from astronomical sources is much less attenuated.

Humans don't really grasp those kind of numbers... nothing we deal with on a day to day basis is measured in quadrillions.

I deal with the I/O end of things, having just installed a file system for another supercomputer that is the fastest (at one TeraByte per second) in the world. A TB/sec is about 170 full length DVD movies per second.

We get these kind of numbers from doing parallel operations... i.e. thousands to hundreds of thousands of processors and disk drives all operating in unison but independently from each other.